The explosive popularity of the Internet has ushered in a new era for telecommunications. The expanding use of applications such as e-mail, online services, secure electronic transactions, media file sharing, video conferencing, remote collaboration, and telecommuting, to name but a few, is causing a significant increase in the bandwidth needs and demands of telecommunications users.
As a result, service providers around the world are continuously searching for economically attractive ways of deploying new high-bandwidth, high-revenue services. Over the last few years, telephone, cable, and wireless operators have been upgrading their equipment in order to utilize their existing infrastructures for higher-bitrate services. The result has been the widespread availability of high-speed data connectivity through Digital Subscriber Line (DSL) service, high-speed digital cable, and, more recently, Third Generation (3G) wireless networks.
Copper telephone lines are generally viewed as the most widely available access medium that is suitable for high-speed data connectivity. In an impressive display of technological progress, the lines that until a few years ago carried only low-speed voice services, are now used to deliver services running at speeds up to several Mbps (Mega-bits per second). This progress has caused the existing copper infrastructure to be viewed as a significant capital asset that telephone carriers can utilize to meet the increasing bandwidth demands of their customers.
Over the last two decades, several technologies for data transmission over copper lines have been developed, including, but not limited to these technologies: T1/E1, ISDN (integrated services digital network), HDSL (High Speed DSL), SDSL (Symmetric DSL), ADSL (Asymmetric DSL), VDSL (Very High Speed DSL) and ADSL2. These technologies have continued to increase the data throughput that can be delivered over copper, but that progress is now being slowed by the shortcomings of the existing copper infrastructure.
Typically, a copper line consists of two copper wires that are twisted together to produce a “twisted copper pair.” Multiple twisted pairs are then twisted together into bundles called “binders.” Twisting of pairs was not used in the early days of voice telephony. It was introduced later in an effort to reduce the effect of interference noise on the received signal. The logic behind this noise reduction is as follows: Interference noise on each copper pair is caused by sources outside the copper pair, such as services operating on other copper pairs in the same binder or in adjacent binders, radio towers, power wires, electrical appliances at the customer's premises, etc. Those sources transmit signals whose electromagnetic fields generate noise voltage signals on each copper wire. If the two wires were not twisted together, the electromagnetic fields from the noise sources could potentially have very different values on each of the two wires, and thus they could generate very different noise voltage signals on each of the two wires. By twisting the two wires together in a copper pair, the electromagnetic fields from the noise sources have roughly the same value on both wires, and therefore they generate noise voltage signals that are approximately equal on both wires. Therefore, when these approximately equal noise signals are subtracted at the differential receiver of the copper pair, the resulting differential noise voltage signal is much smaller than it would have been had the two wires not been twisted. On the other hand, the main communications signal, which is transmitted as the voltage difference between the two wires of the copper pair, is not affected by the twisting of the wires and is received by the differential receiver in full strength. Therefore, the use of twisted copper pairs and differential receivers reduces the effect of the noise only, and does not have a similar reduction effect on the main signal. As a result, the SNR (Signal-to-Noise Ratio), and thus also the communications capacity, that can be achieved on a twisted copper pair with a differential transmitter and receiver is substantially higher than the SNR of an untwisted copper pair of the same gauge and length with a common-mode (i.e., non-differential) transmitter and receiver.
When telephone carriers began setting the specifications for the copper pairs used in their cables, including the gauge and twisting of the wires, they considered only POTS (Plain-Old Telephone Service) voice services, which typically occupy only a small frequency band at the low end of the spectrum, approximately from 0 to 4 kHz. As a result, they decided to use relatively thin wires, mostly 26AWG and 24AWG, and to twist them once every few feet or so. At the time, these choices of gauge and twist length represented an excellent tradeoff between performance and cost. Wires of larger gauges that are twisted more tightly cost more to produce and are heavier, larger, and less flexible; therefore, they also cost more to transport and to install. Moreover, having enough copper pairs to serve all the current and future customers in a given area means that, as the wires become larger and heavier, the underground conduits that carry them have to be larger, and the poles that support them have to be sturdier. Since POTS service utilizes frequencies below 4 kHz, these choices of gauge and twist length were perfectly adequate for ensuring high-quality POTS service with very low crosstalk noise. Crosstalk originally denoted the noise that was generated when the voice of one telephone subscriber talking on his line “crossed” into another subscriber's line and could be heard as low-volume background speech.
Unfortunately, these choices do not guarantee the same performance when the copper pairs are used to deliver high-speed data services, which typically operate at frequencies that are more than 100 times higher than POTS frequencies. The small gauge of the wires results in high attenuation of high-frequency signals; as a result, the data capacity of a copper pair decreases rapidly as the length of the pair increases. Even worse, the long twisting of existing copper pairs is much less effective at reducing crosstalk noise at higher frequencies. As the frequencies used for transmission and reception of signals rise, the wavelengths of those signals are reduced. Therefore, pairs with long twisting appear more and more as untwisted pairs to the corresponding short-wavelength electromagnetic fields. Since the cost of replacing existing copper pairs with new ones that have much shorter twisting would be prohibitively high, the only economically viable option is to use existing pairs for the transmission of high-speed data services. As a result, high-speed data services are much more sensitive to crosstalk noise from outside sources. This is especially detrimental on longer lines, since the main signal on those lines is already significantly attenuated; as a result, the increased crosstalk noise often reduces the SNR to levels that are not suitable for high-speed data transmission.
The realization that crosstalk is one of the primary causes of performance degradation in high-speed data transmission over copper pairs has resulted in substantial attention to the problem of mitigating crosstalk. Transmission technologies designed for operation over a single copper pair attempt to mitigate crosstalk through the use advanced coding schemes and adaptive filters that aim to maximize SNR in a given crosstalk environment. Such mitigation techniques have evolved steadily over the last decade, and have reached a level of maturity where small additional SNR gains come at the cost of significant increases in complexity.
The most recent advance in the effort to mitigate crosstalk on copper pairs is the use of “vectoring” in multiline transmission schemes. In such schemes, multiple copper pairs are used to deliver high-speed services; but instead of simply using each copper pair as a separate communications channel and “bonding” the corresponding data streams at the digital layer, vectoring techniques coordinate the transmission and/or reception of signals at the physical layer, in order to increase the overall capacity of the multiline communications channel. One such vectoring scheme, disclosed in a recent application PCT/US 03/18004, which is incorporated herein by reference, exploits the correlation of crosstalk noise across its associated multiple copper pairs. In particular, that scheme treats the transmitters and receivers on multiple copper pairs as inputs and outputs of a MIMO (Multiple Input Multiple Output) communications channel. Operating in the signal space defined by these multiple inputs and outputs, the scheme identifies the subspace that contains the crosstalk noise, and then uses MIMO pre-processing at the transmitter and MIMO post-processing at the receiver to transmit most of its main signal in the subspace that is orthogonal to the crosstalk noise. For DMT (Discrete-Multi-Tone) systems, these operations are simply matrix multiplications in the frequency domain. The MIMO post-processing at the receiver consists of multiplying the received symbol vector in each bin by a matrix that combines the operations of noise pre-whitening and frequency equalization. The MIMO pre-processing at the transmitter consists of multiplying the transmitted symbol vector in each bin with a matrix that compensates for the distortion caused by the noise pre-whitening matrix at the receiver. As a result, the main signal is received without distortion, while the crosstalk noise, which is not multiplied by the MIMO pre-processing matrix, is restricted to a small subspace of the received signal space.
Another way of interpreting the vectoring effect of that MIMO scheme is that it identifies the crosstalk noise on some of its receivers and then removes it from the remaining receivers, thereby significantly reducing its overall effect. As a rule of thumb, the effectiveness of this type of crosstalk mitigation is reduced as the number of strong independent crosstalk sources increases beyond the number of receivers available in the multiline system. This is due to the fact that each independent crosstalk source increases the dimension of the crosstalk subspace by one. As the number of such crosstalk sources increases beyond the number of receivers available, the dimension of the crosstalk subspace becomes equal to the dimension of the signal space of the multiline system, thereby eliminating the orthogonal subspace where the main signal can be received free of crosstalk noise.
Therefore, it would be desirable to find a way to increase the dimension of the signal space in a multiline system for a given number of copper pairs used.